1. Field of Invention
The invention concerns means and methods which enable the provision of plants showing an altered flowering behaviour and, particularly, genetically modified plants showing an early or delayed flower formation when compared to wild-type plants, i.e., not correspondingly modified, but otherwise similar plants. The invention also pertains to a tissue-specific promoter which may be utilized in a method for producing the transgenic plants according to the present invention.
2. Description of Background
In plants, the formation of flowers is a prerequisite for the sexual reproduction. Therefore, it is essential to the reproduction of plants incapable of vegetative reproduction, as well as for the formation of seeds and fruit. The time at which plants undergo the transition from merely vegetative growth to flower formation is of great importance, for example, in agriculture, horticulture and plant breeding. As well, in many cases, the number of flowers is of economic interest, e.g., in case of various useful plants (such as tomato, cucumber, zucchini, cotton, . . . ) where a higher number of flowers will possibly result in an increased yield, or in the production of ornamental plants and flowers for cutting.
In many applications, a very early flower formation of plants will be advantageous. In agriculture, for example, early flowering of various useful plants may reduce the time between sowing and harvesting and, thus, enable sowing two times a year. Alternatively, the time between flowering and harvesting may be extended and, as a result, the yields will potentially increase. As well, in plant breeding, an early flower formation may contribute to a significant shortening of the breeding process and, thus, result in an improved economic efficiency. The economic benefit of an early flowering is also evident in horticulture and in the production of ornamental plants.
The attempts made to date for elucidating the mechanisms determining the time of flower formation in plants do not allow an unambigous conclusion with respect to the decisive factors therefor. Various factors appear to be involved in a probably highly complex biological system (Bernier, 1988, Ann Rev Plant Phys Plant Mol Biol 39: 175-219). It is known for a number of plants that environmental influences determine the transition from vegetative growth to flower formation, such as light-dark cycles, temperature and water supply. Although it is known how the perception of these stimuli is effected by the plantsxe2x80x94for this, light receptor proteins of the phytochrome system are responsiblexe2x80x94it is unknown how the stimulus is converted or translated into physiological signals which induce the flower formation in the apical meristem. Various theories are discussed and quite a number of potential factors are taken into consideration such as flowering hormones (florigen/antiflorigen), carbohydrates, cytokinins, auxine, polyamines and calcium ions (Bernier et al., 1993, Plant Cell 5: 1147-1155).
In practice, the control of the time of flower formation by regulation of exogenous stimuli such as light-dark cycles, temperature or water supply, may be achieved only to a limited extent, for example in greenhouses. To achieve an early flower formation in plants growing under field conditions it is therefore necessary to use plants which show an early flower formation irrespective of exogenous stimuli. Plants showing early flowering may be produced by mutagenesis procedures which, however, are not applicable to all species, by plant breeding processes which, however, are very time-consuming and have to be carried out separately for every particular plant species, or by genetic engineering.
A prerequisite for the applicability of genetic engineering is that gene loci have been identified which have a significant influence on the time of flowering, and that DNA sequences are available encoding the relevant products. At present, the gene product of the CONSTANS gene from Arabidopsis thaliana is known. The expression of this gene causes the onset of flowering in this species. However, a selective overexpression of this gene in transgenic plants in order to influence the flowering behaviour, will not be technically feasible since a constitutive expression of the gene will lead to a severe shortening of the vegetative growth phase of the transgenic plants and, therefore, will reduce the yields. Simon et al. (1996, Nature 384: 59-62) describe an inducible system for the expression of CONSTANS which cannot be technically used since the use of steroid hormones which cause the induction, is unacceptable in agriculture.
For the species Arabidopsis thaliana which has been the subject of most investigations on the regulation of the time of flowering, various mutants flowering early in comparison with wild-type plants have been described (see references in Lee et al., 1994, Plant Cell 6: 75-83). However, it has not yet been possible to characterize these mutants. As well, the elucidation of the biochemical causes leading to the early flower formation has not yet been successful.
Bell et al. (1993, Plant Mol. Biol. 23: 445-451) describe tobacco plants transformed with cdc25 cDNA from Schizosaccha-romyces pombe which show an early flower formation and a strongly increased number of flowers due to the expression of this mitose-inducing protein. However, these plants are disadvantageous in that severe changes of the leaf morphology are observed. Particularly, the leafs of these plants are curled. Therefore, this method appears to be unsuitable for producing useful plants showing an altered flowering behaviour.
For producing intact plants showing an early flower formation, one has still to rely on classical breeding procedures or on methods utilizing mutagenesis. Any function of saccharose-cleaving proteins in the regulation of the flowering behaviour has not been considered yet.
It is true that saccharose has been repeatedly discussed as a potential component of a complex signal for the flowering induction in the apical meristem (Bernier et al., 1993, Plant Cell 5: 1147-1155; Lejeune et al., 1993, Planta 190: 71-74; Lejeune et al., 1991, Plant Physiol. Biochem. 29: 153-157); however, any influence of a saccharose-cleaving protein on the flowering behaviour is neither yet known nor to expect since according to findings of Lejeune et al. (1993, Planta 190: 71-74), in some species, an increase in the saccharose concentration at the apex is associated with the induction of flowering.
Furthermore, the expression of saccharose-cleaving proteins in transgenic plants usually has strongly detrimental effects for the growth of said plants. An expression of the enzyme invertase from Saccharomyces cerevisiae under the control of a constitutive promoter in tobacco plants causes necrosis in the leaves irrespective whether the protein is localized in the apoplast, cytosol or vacuole (Sonnewald et al., 1991, Plant J. 1: 95-106).
The problem underlying the present invention is to provide means and methods which make it possible to alter the flowering behaviour of plants and particularly to provide genetically modified plants which show an early or delayed flower formation compared to not-modified plants.
This problem is solved by means of the transgenic plants, promoter elements, constructs, transformed host cells, processes and uses as defined in the attached claims.